This is the second time Dr. Corsi has been involved in an effort to question the credentials of the Democratic presidential candidate. The episode also underscores the depth of support for Senator Obama in Kenya.

His book has angered many ordinary Kenyans who view Obama – the son of a Kenyan goat herder who grew up to become a government economist – as a hero.

It questions Obama’s fitness to hold office, rakes over his family history, and explores alleged links with the Kenyan prime minister, Raila Odinga.

“We are going to leave for London tonight,” said Corsi’s assistant, Tim Bueler, on Tuesday afternoon.

“Our passports and visas are in order but it seems the Kenyan authorities have lost our arrival cards, which we filled in on the plane coming here,” he said. “So we will leave while they complete their investigations, but have been told we can return.”

As well as launching his book, Corsi planned to visit Obama’s half-brother George, who was recently found to be living in squalid conditions in a Nairobi slum.

Corsi, who wrote “Unfit for Command” in 2004 as part of an attempt to malign John Kerry’s Vietnam record, said he had a check for $1,000 to give to George.

The writer has been in Kenya since last Thursday.

But it seems the Kenyan authorities woke up to his presence on Tuesday morning when local newspapers advertised the book launch.

The Standard, a Kenyan newspaper, quoted a press release saying: “Dr. Corsi will also expose details of deep secret ties between US presidential candidate Sen. Barack Obama and a section of Kenya government leaders, their connection to certain sectoral groups in Kenya and subsequent plot to be executed in Kenya should Sen Obama win the American presidency.”

Obama has ancestral roots in the Luo tribe of Western Kenya, as does Mr. Odinga.

Raila narrowly lost December’s presidential election which was marred by widespread vote rigging. His defeat prompted widespread violence which left some 1,500 dead. It ended only with a power-sharing deal that promoted Odinga to the new position of prime minister.

Obama is deeply popular in Kenya, where he is seen as a political hero, untainted by the corruption and sleaze that dog homegrown politicians here. He has attracted broad support that overcomes traditional tribal divisions.Corsi’s book alleges that Obama was raised as a Muslim, a false rumor.